Seismic amplifying system



' Nov. 18, 1947. H. HOOVER, JR 2,430,983

V SEISMIC AMPLIFYING SYSTEM Original Filed July 24, 1959 5 Sheets-Sheet1 4 ME INVENTOR.

Hffifi7' HOOVIR, JR,

Nov. 18, 1947. H. HOOVER, JR 2,430,983

7 SEISMIC AMPLIFYING SYSTEM i Original Filed July 24, 1939 5Sheets-Sheet 5 i INVENTOR. i HIP/762777 Ham/[A Patented Nov. 18,1947

UNITED STATE OFFlCE SEISMIC AMPLIFYING SYSTEM Griginal application July24, 1939, Serial No.

Divided and this application November 24, 1941, Serial No. 420,194.

3 Claims.

My invention relates to geophysical prospecting, and more particularlyto circuits which control the sensitivity of seismic recordingapparatus. My invention is applicable, as well, to any other signallingsystem in which a train of waves of varying intensity is received, andis especially applicable to systems utilizing wave-trains with suddenbeginnings. This application is a division of my copending patentapplication Serial No. 286,171, filed July 24, 1939, now Patent No.2,395,481, issued February 26, 1946.

Among the objects of my invention are: to provide means for varying thesensitivity of seismic recording apparatus during recording; to providea means for providing intermediate sensitivity in recording thebeginning of a wave-train; to provide means for reducing the sensitivityin recording seismic waves shortly after the beginning of wavereception; to provide means for increasing the sensitivity toward theend of the recording when the waves which have traveled the longest timeare received; to provide means for producing a quiet trace preceding thebeginning of the Wave-train; to provide means for varying thesensitivity of an amplifier as a predetermined function of time; toprovide a control unit which, once started, thereafter controls variousoperations in a predetermined time sequence; to provide means forvarying the sensitivity of an amplifier as a predetermined function oftime and to change the time function at predetermined time intervals; toprovide means for indicating the variation of seismic wave amplifiersensitivity during recording; and to provide means for preventing crossfeed between a plurality f amplifiers utilizing a common gain controlunit.

Other objects of my invention will be apparent or will be specificallypointed out in the description forming a part of this specification, butI do not limit myself to the embodiment of the invention hereindescribed, as various forms may be adopted within the scope of theclaims.

Referring to the drawings:

Fig. 1 is a schematic diagram illustrating the application of myinvention to seismi prospecti Fig, 2 is a reproduction of a field recordused in illustrating the application of my invention to seismicprospecting.

Fig. 3 is a graph useful in explaining the operation of my invention.

Fig. 4 is a wiring diagram of one embodiment of my invention.

Fig. 5 illustrates another embodiment of my invention.

Fig. 6 illustrates still another embodiment of my invention.

In Fig. 1 I have illustrated schematically a setup by means of which myinvention may be applied to seismic prospecting. U represents a verticalcross-section of the earth near its surface, and W represents theweathered layer.

In the practice of seismic geophysics, elastic waves are initiated at asource E, usually by the detonation of an explosive at the bottom of ashot hole H. When the waves originating at the source E, which areradiated in all directions as indicated by the lines P, encounterdiscontinuities in the earth they are deflected from straight paths, asby refraction or reflection. Some of the waves returning to the surfaceof the earth are received by receptors S1, S2, and S3 which convert thereceived elastic waves into corresponding electrical waves. Theelectrical Waves are amplified by amplifiers A1, A2, and A3, andtranslated into visual form by multielement galvanometer O. The wavesfrom the seismometers may be recorded as separate traces on photographicpaper.

Part of a record obtained by using six recording channels in accordancewith the principles of my invention is shown in Fig. 2. For conveniencethe traces are referred to as It to St, beginning from the bottom.Vertical timing lines are represented by t. T. B. on the second trace isthe time break, which indicates the instant the wave source E isactivated, and F. B. represents the first break for a wave-trainreaching the receptor, the output of which is recorded on trace 3t. Saidfirst break determines the instant of arrival of the initial portion ofthe wave-train at said receptor.

It is to be noted that the first breaks on the various traces are sharp,and easily readable. The waves immediately following the first breaksare very large, in this case, in fact so large on some traces that theycannot be studied easily. As is readily seen from the record the firstwave to arrive is generally small compared with the 3 waves immediatelyfollowing and for this reason a rapid decrease in sensitivity isnecessary to provide for recording the early portions of each tracefollowing the first arrivals, while providing for the recording of asharp first break,

As can be seen, the average amplitude of the subsequently recorded waveson the rest of the record is favorable for the identification ofcorresponding WaVes appearing on the Various traces.

It is also to be noticed that the traces are quiet, or free from noise,or disturbing microseisms, preceding the first breaks.

I achieve quiet traces and sharp first breaks by using intermediatesensitivity in my ampli: fiers during the early part of the recording.If the gain were low the first breaks would be small and therefore notso easily identified; if the gain were high microseisms or unrest wouldbe reproe duced at sizeable amplitude and would interfere with therecognition of first breaks.

To achieve easily readable waves shortly following the first breaks andthereafter, I reduce the amplifier sensitivity shortly after the firstbreaks are recorded and thereafter vary the sensitivity as apredetermined function of time in such a manner .as to compensate forthe varying amplitude of the received waves.

Actually, of course, the entire record of which Fig. 2 is a part extendsto the right. fora considerable length corresponding to atotal recordingtime of four or five seconds or more and usually contains toward the endrecorded waves of very low intensity.

I prefer to increase the sensitivity continuously toward the end of threcord until'the amplitude of the unrest completely masks any reflectedwaves that may be present. Inthis manner I am assured of obtaining amaximum amount of information from each shot,

In one preferred form of my invention the first breaks may be made sharpon each trace, and the average amplitude. of the following waves asrecorded on each trace maintained between limits suitable for study. Iobtain this result by using as high gain as possible in my amplifierswhile the first refracted. waves are being received, without making-thegain so high as to record earth unrest; and thereafter vary the gain ofthe amplifiers in accordance with the amplitude ofthe received waves.

In some areas sharp. first breaks may be obtained by using low gain atthe beginning ofthe recording. In such cases my apparatus may be readilyadapted to provide a sensitivity vs. time function such as thatrepresented by curve a of Fig. 3. In some cases the discharge of acondenser through a single resistor mayprovideall the gain controlnecessary but thissimple method is inadequate in many areas where itsapplication inevitably results in excessive amplitudes during someportions of the recordings and small amplitudes in other portions of therecording. Usually it is not possible to compensatefor thetime-amplitude pattern of received seismic waves in such a simplemanner. To obtain good records of waves from various depths below thesurface in such'a casev it accordingly is necessary to make a number ofrecords successively for each set-up using differing settings of thegain control device.

My invention overcomes these difficulties by providing .a method formeeting a wide variety of conditions by making possible the changing ofthe time vs. sensitivity curve at predeterminedspaced time intervalsduring the recording,

In Fig. 3 curve bod represents graphically one 4' manner in which Iprefer to vary amplifier sensitivity during the recording. The instantsat which the time break 'I. B, and the first break F. B. occur arerepresented by T1 and T2, respectively. T3 and T4 represent instants atwhich the time sensitivity function is changed.

In Fig. 4 I have illustrated one particular preferred arrangement ofapparatus which may be operated in accordance with my invention.

The control unit C has two main sections, timing unit I and sensitivitycontrolling device 2.

As illustrated, the timing unit I utilizes three grid controlled gaseousdischarge tubes l3, I 3, and

15, relays l8, H, and i2, and time dela circuits 20 and 2!. To set thetiming unit I preparatory to operation, the key i5 is temporarilyopened, thereby stopping the currents in the plate circuits of the gridcontrolled gaseous discharge tubes l3, Ml, and ill and causing thecontacts of the relays lil, II, and E2 to be closed in the leftpositions as shown. A push-button 4? normally open may be placed in thegrid circuit of tube I3 for purposes hereinafter to be explained. Thetubes referred to herein as grid controlled gaseous tubes are theordinary well known type of gas filled triode ordinarily having a hotcathode, an anode, and a control grid for controlling the dischargethrough the tube.

A signal derived from some suitably energy source, such as the currentwhich detonates the explosive E, is applied to the primaries oftransformers, 28 and 23. A corresponding voltage appears in thesecondary of transformers 26 and 28 and, if the connections arepolarized properly, a positive potential is thereupon applied to thegrid 4 of the gaseous tube l3. Ihis positive po tential is made largeenough to fire said gaseous tube It, Potentiometer 29 controls thesensitivity of gaseous tube I3.

At the instant current is applied tothe-primary 26 a correspondingsignal may be applied to a galvanometer 2'! which records the time breakT. B. of Fig. 2 and occurs at the instant T; of Fig. 3.

When the gaseous tube 53 is fired, the relay It .opens and the condenser22 of the timing circuit 20. begins to discharge through the resistance23, at a rate depending on the time constant of the timing circuit 20.At an instant T3 of Fig. 3 the potential on the grid 5 swings in thepositive direction enough to fire the gaseous tube l4, causing relay Hto open in the left position'and close in the right position.

When the pair of contacts ll is opened, the condenser t l dischargesthrough the resistance 25 at a rate depending von the time constantofthe timing circuit 2!. Finally at the time T4 the bias on the grid 6swings. in the positive direction enough to permit the. gaseous tube 5to fire, and the relay i2 is thereby caused to open in the leftpositionand close in the right position.

A greater number of control stages than those shown may be used in thetiming unit E to provide greater flexibility. in the determination ofthe sensitivity vs. time curve. An unlimited variety ofshapes of thesensitivity curve may thus be obtained by approximate application of myinvention. I

The gain controlling device 2;; may comprise a triode 3G, with grid 35,plate 5 2, and, cathode 33. In the plate circuit is a voltmetertfi across which potentials appear in accordance with the plate current. Saidvoltmeter is conveniently placed in the recorder truck to aid anoperator to visualize the gain control action of control unit 0 Asillustrated, the plate 32 is grounded at 34 and the potential of thecathode 33 allowed to vary at a value below ground potential. Thepotential existing across the voltmeter 35 may be applied to theamplifier A in any suitable manner known to those skilled in the art tocontrol the sensitivity of said amplifier A. In the example shown thecontrol potential across voltmeter 35 supplies voltage to screens 50, 5|of vacuum tubes within amplifier A.

Since the current through voltmeter 35 is determined by the bias on grid3 I, said .bias accordingly determines the sensitivity of the amplifierA.

To vary the bias of grid 3| of the triode 3D, I use a battery. acrosswhich are connected potentiometers, 42, and 43. For convenience thepotentials between the cathode 33 and the sliding contacts 6|, 62, and63 may be referred to as the first, second, and third biases,respectively. When the control unit C has been set in a ready conditionby opening the key l6 temporarily, as hereinabove the relays H and I2are closed in the left position as shown in Fig. 4. In the readyconditions, therefore, the grid 3| is connected to the potentiometer 4|through contacts in the relays II and i2. The first bias is thereforethe initial bias.

Now consider a typical sequence of operations which may occur during therecording of a train of seismic waves. Assume that the first, second,and third biases, hereinabove referred to, are respectively, ofintermediate, high, and low values, as indicated by the positions of therespective contacts 6 I, 62, and 63 on the potentiometers 4|, 42, and43.

If an explosive is detonated at E at a time T1 the timing unit I isactuated, the first break F. B. is recorded at time T2 while thesensitivity is a steady intermediate value indicated by section b of thesolid curve in Fig. 3. At time T3, shortly after recording the firstbreak, relay opens in the left position and. closes in the rightposition. Current from battery 40 begins to flow through resistor 44charging condenser 46, thereby increasing the bias on grid 3| anddecreasing the sensitivity of amplifier A as a function of time such asalong section of the solid curve in Fig. 3.

At time T4. relay I2 opens in the left position and closes in the rightposition. Condenser 46 discharges through resistor 50 and 43, and thesensitivity of amplifier A increases along a curve such as d of Fig. 3.The values of resistances 44 and 50 and condenser 46 may be adjusted tovary the shape of the sensitivity curve of Fig. 3.

In practice I generally make condenser 31 large to prevent sharp pulsesfrom being suddenly applied to screens of tubes 52 and 53. Condenser 31also serves to decouple a plurality of amplifiers using a common controlcircuit and thereby prevents crossfeed. Screen grid condensers 10 and 1|in various amplifiers using a common control circuit also aid inisolating the amplifiers from each other.

By means of other relays operated in the manner hereinabove explained,further changes in the amplifier sensitivity as a predetermined functionof time may be made at later times such as T and T6.

In the form of my invention illustrated in Fig. 1, the sensitivity ofall amplifiers is governed by a common control unit and the operation ofsaid control unit is initiated simultaneously with the detonation of anexplosive at E. But other forms of my invention may be adopted.

For instance, the operator of the seismic Wave recording apparatus mayinitiate the operation of the control unit C manually by closingpushbutton 41 at the proper time. By watching the galvanometer spotsthrough a window in oscillograph 0, he may observe the arrival of thefirst waves and close push-button 41 immediately thereafter. In somecases he may close pushbutton 41 shortly after closing firing key K byproperly timing the operation.

Another form of my invention is illustrated in Fig, 5. This represents amodification of the circuit of Fig. 4 and only so much of the circuit isshown as necessary to explain the operation and indicate the differencesbetween Figs. 4 and 5.

In Fig. 5 the output of amplifier A is applied to a vacuum tube l3through transformer 28, and a sticking relay i3 is used in the platecircuit of said tube I3.

When the first waves arrive at the receptor connected to the input ofamplifier A, corresponding electrical waves appear in the output of saidamplifier. Part of said output is applied to a recording galvanometer Gand part is applied to tube I3, causing said tube to close relay l0.When relay I0 is closed condenser 22 in timing circuit 20 discharges andrelays II and I2 are closed in turn a hereinabove explained. In the caseof Fig. 5, however, the extra pair of contacts 49 are closed and thepotential of battery 48 is applied to the coil of relay ID to hold itclosed during the remaining time while control unit C varies thesensitivity of amplifier A.

Fig. 6 is a schematic diagram of the method of Fig. 5 as I apply it to aplurality of amplifier channels separately. Here receptors S1, S2, andS: are connected to their respective amplifier A1, A2, and A3 and theirrespective recorders O1, O2, and O3. Amplifiers A1, A2, and A3 are eachcontrolled by separate units C1, C2, and C3 in the manner described inconnection with Fig. 5. When separate control units are applied to eachrecording channel I prefer to use ordinary vacuum tubes at l3, l4, andIE to cut down on the filament drain required and to provide a highizigipedance across the secondary of transformer Initiation of thesensitivity control operation by the first waves to arrive at therespective receptors facilitates obtaining sharp first breaks on eachtrace even though the receptors be spaced several hundreds or thousandsof feet apart and the amplifiers and recorders are established at therespective receiver stations.

Thus it is seen that I may initiate the operation of the sensitivitycontrol unit manually, or automatically by the first arrivals or byenergy derived directly from the wave source, and I may apply suchcontrol to all amplifier units separately or jointly.

From the foregoing description it is clear that I have provided a methodfor obtaining sharp first breaks in translating seismic wave-trains andat the same time have supplied a method for controlling the sensitivityof the receiving apparatus in a manner which compensates for variationsof the amplitude of waves following the first breaks.

mined sequence to. obtain a desired sensitivity voltage variation acrosssaid condenser.

2. In combination with an amplifier having a vacuum tube provided with acontrol element the D. C. voltage on which determines the gain of theamplifier and mean for biassing said element from a source of D. C.voltage; connections adapted. to apply different values of voltage fromthe source to the control element and timed control means for selectingsaid connections, said control means comprising tandem-arranged relaysand timing circuits between successive relays whereb the successiverelays are energized at predetermined time intervals from the precedingrelay and means for selecting a difierent one of the connections uponenergization of each successive relay.

3. Apparatus according to claim 2 in which the relays are gridcontrolled gaseous tubes and each timing circuit comprises a resistanceand condenser.

HERBERT HOOVER, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

Number .UNITED STATES PATENTS Name Date Bruce June 13, 1933 Threlkeld'June 13, 1933 Place May 15, 1934 Elliott Mar. 30, 1937 Snow et al. Aug.16, 1938 Prescott May 16, 1939 Minton May 30, 1939 Koch Feb. 6, 1940Mufily Oct. 15, 1940 Braden Dec. 24, 1940 Allen Feb. 18, 1941 PaslayNov. 25, 1941 Ritzman'n May 5, 1942 Heiland June 9, 1942 Shock Mar. 2,1943 Peters May 11, 1943 Weatherby et al. June 8, 1943 Woods 1- Dec. 7,1943 Minton July 25, 1944 Cloud May 8, 1945 Scherbatskoy May 15, 1945Rieber June 12, 1945

